1 mm cubical voxels at 3 Tesla The following images show the results from two functional MRI experiments:

• The first with voxel dimensions 1x1x10 mm**3 and 1 slice;
• The second with voxel dimensions 1x1x1 mm**3 and 10 slices.

Both experiments used FOV=128 mm, image matrix=128x128, and TR=2 s. The background images are the 10 slices gathered in the first EPI shot of the high resolution experiment. With this contrast, CSF is the brightest, gray matter next, and white matter slightly darker than gray matter. The task alternation is 40 s rest and 40 s bilateral finger tapping, for a total of 150 acquisitions (300 s). The function is calculated from all slices, and rendered into 3D using AFNI.

Left: 1x1x10 mm**3 data. Right: 1x1x1 mm**3 data.

At the start of the animation, the 10 mm thick slab is viewed from above. The color overlay that then appears represents the functional activation, with red indicating signal changes under 15% and yellow signal changes over 20%. The background images are rendered as partly transparent so that the color overlay can "shine through". As the animation progresses, the viewpoint swings around to the side, showing the function from different angles. The background image then fades out completely, and the activated volumes are left hanging in space while the viewpoint swings back to the original top-down orientation.

On the left, the activated regions are crudely shaped along the inferior-superior axis, due to the low spatial resolution in that direction.

The color shows that the estimated signal change due to activation is smaller in the low resolution dataset. This is probably due to partial-volume effects: not all of the volume of a 1x1x10 mm**3 voxel will be filled with "activated" tissue. Since the signal from a voxel is averaged over its entire contents, if only a small portion of the voxel has a large signal change, the net measured result is a small signal change. With smaller voxels, the regions that are not active will be pruned away and the observed signal changes will be larger.